Recently, various sized optical elements from large sized ones such as those to be use for telescopes and microscopes to small sized ones such as those to be used for optical pickup lens for compact disc (CD), digital video disc (DVD), high density DVD (HD-DVD) and Blu-ray Disc (BD) are produced and on the market.
Such the elements are molded by ejecting a resin specified for the use into a designated metal mold, so called an ejection mold method. In such the case, double refraction causes on the surface layer of the molded product by remaining stress and scattering of light occurs in the surface layer so that the focal distance of the product is difficultly adjusted to the required designed value.
It is disclosed that the double refraction caused in the product can be minimized by relaxing the remaining stress in the molded product by annealing the product after cooled at a temperature lower than the glass transition temperature; cf. JP-B No. 3055443, for example.
Moreover, a method is known in which a resin containing a polymer having an alicyclic structure is molded by ejecting into a metal mold and the molded resin is treated by heat treatment or wet-heat treatment for 16 or more hours at a temperature lower by 35 to 45° C. than the glass transition temperature of the resin; cf. JP-A No. 2005-275393, for example.
However, the technique described in JP-B No. 3055443 is a method for providing the annealing treatment on the occasion of molding of a relatively large optical element for giving suitable properties to the molded product. It specifies the annealing conditions in detail, but the conditions is hardly applied for entire resin optical elements produced by the ejection molding process. For instance, in the case of small sized optical elements having a size of several millimeters to be used for the optical pickup lens, the light-spot forming ability is largely influenced by the difference in the annealing condition when the annealing condition is slightly different by every production lot.
Therefore, the remaining stress cannot be sufficiently relaxed and the thermal equilibrium condition cannot be stably kept during the annealing treatment by the annealing treatment so that the sufficient properties as the optical element for the optical pickup lens is hardly obtained by the technique disclosed in JP-B No. 3055443.
In the technique disclosed in JP-A No. 2005-275393, the annealing conditions for sufficiently relaxing the remaining stress in the small sized optical element and greatly improving the optical properties such as astigmatism are minutely specified. However, an optical element having flat-plate like shape or an optical element thinned and small sized in diameter such as that for HD-DVD is easily deformed by heat and a high NA optical element such as for BD is very sensitive so that the optical property thereof is easily influenced by slight deformation. Therefore, the conditions described are not entirely corresponded to the properties of such the optical elements.